HOLOCENE FREQUENCY OF PALEO-FLOODS IN MOBILE BAY FROM SEDIMENT GRAIN SIZE AND ELEMENTAL ANALYSES: IMPLICATIONS FOR RECENT ESTUARINE EUTROPHICATION EVENTS

Flood events are known to cause salinity changes, increase nutrient and sediment flux, and trigger harmful algal blooms (HABs) that may lead to bottom water hypoxia in coastal bays. HABs can cause seafood “Jubilees,” in which neritic species swim to shallow environments to escape hypoxic conditions. Although Jubilees have been celebrated for the easy-to-catch seafood, they are a symptom of damage to ecosystem health, with consequences for local aquaculture and human health. Study of the Holocene record prior to European settlement and industrialization is critically needed to determine the natural record of floods, nutrient flux, and HABs and to provide a baseline for evaluating the severity of anthropogenic impacts.

Our study tests the hypothesis that riverine flood events have a sedimentary and geochemical fingerprint in coastal estuarine basin stratigraphy. We aim to develop multi-proxy paleo-flood assessment tools for coastal bays and evaluate the frequency of riverine floods in Mobile Bay, AL over the last ~8 kyrs. Piston Core MB 18-01 was collected in north-central Mobile Bay and recovered 8.39 m of Holocene bay mud. Laser particle size distributions, measured at cm increments throughout with a Malvern Mastersizer 3000, show variability in sand concentration, with peaks indicating likely paleo-flood events from the Mobile-Tensaw River Delta. Corresponding cm-interval sampling of elemental composition, measured with a handheld SciAps X-Ray Fluorescence device, shows variability in Al, Ti, K, Zr that correlates with changes in sand concentration and supports the interpretation that flood events can be recognized in bay stratigraphy. Foraminiferal assemblages will be studied to elucidate relationships of flood events with eutrophication (nutrient loading that leads to HABs) and hypoxia. Results will be integrated with published basin analyses that document Holocene bayhead delta backstepping events and recent hydrographic changes resulting from shipping channel construction. Radioisotope ages from MB 18-01 and nearby cores will be employed to compare the recent, human-influenced time period within a millennial context to: 1) evaluate impact of agriculture and infrastructure on the bay and 2) provide a framework for understanding the impact of industrial pollutants on coastal ecosystems.